High power semiconductor lasers are key components for many telecommunication and industrial applications. An important characteristic of a semiconductor laser is the divergence of its output beam. The lower the divergence, the easier it is to deliver the beam to a target, or to efficiently couple the beam into an optical fiber of a fiber-coupled semiconductor laser.
The output beam of a side-emitting semiconductor laser has more divergence in a vertical direction, i.e. in a direction perpendicular to the active layer plane, compared to the divergence in a horizontal direction, parallel to the active layer plane of a semiconductor laser. A more circular beam enables one to achieve more efficient coupling into an optical fiber. A small beam divergence angle can also improve the fiber alignment tolerance, which is important for maintaining coupling efficiency stability in the long term.
There have been many efforts to reduce the vertical far-field beam divergence of a semiconductor laser; however, most of them either were at lower output power, or resulted in poor performance, such as high threshold current or low slope efficiency.
Semiconductor lasers are typically based on p-n junctions of a p- and n-type semiconductor layers, which is why semiconductor lasers are frequently called laser diodes. The p-n junction of a laser diode is enhanced to facilitate the emission of photons due to recombination of electron-hole pairs. A well-known way to achieve such an enhancement is to add a new layer at the p-n junction, the new layer having a lower bandgap energy than p-type and n-type layers abutting it. The layer is commonly referred to as the active region of a semiconductor laser. An optical mode of the laser is primarily confined in the active region because of the difference in the index of refraction between the active region and the p- and n-doped layers. The active region provides gain to the optical mode when the p-n junction is forward-biased. The active region itself is often composed of many layers in order to tailor the performance of the laser to meet the requirements of a particular application.
One approach to reduce the vertical far-field divergence of a laser diode is to keep the layer structure symmetrical, and to design the layers so as to expand the optical mode in a direction perpendicular to the layer planes, thereby reducing the far-field divergence of emitted light in said direction. For example, in an article by G. Lin, S. Yen, C. Lee, and D. Liu entitled “Extremely low vertical far-field angle of InGaAs—AlGaAs quantum well lasers with specially designed cladding structure”, IEEE Photo. Tech. Lett., vol. 8, pp. 1588-1590, 1996, the authors describe using two inserted low index layers between the active region and p- and n-cladding layers to expand the optical mode. In a U.S. Pat. No. 5,197,077 by Harding et al., a symmetrical layer structure of a laser diode is described consisting of triple graded-waveguide multi quantum wells, the center quantum well serving as a lasing region, and the other two quantum wells acting as optical trap layers for expanding the optical mode. Further, in an article by M. C. Wu, Y. K. Chen, M. Hong, J. P. Mannaerts, M. A. Chin, and A. M. Sergent, entitled “A periodic index separate confinement hetero-structure quantum well laser,” Appl. Phys. Lett., vol. 59, pp. 1046-1048, 1991, the authors describe periodical super-lattice-like layers, introduced in the cladding layer of the symmetrical structure to effectively reduce the index contrast and expand the optical field, which reduced divergence of the beam in a direction perpendicular to the layer planes. Yet further, in an article by S. T. Yen and C. P. Lee entitled “Theoretical investigation on semiconductor lasers with passive waveguides,” IEEE J. Quantum Electron., vol. 32, pp. 4-13, 1996, the authors investigate using two high index passive waveguides, inserted in the p and n-cladding layers to expand the light mode for smaller beam divergence. All the articles and patents mentioned in this paragraph are incorporated herein by reference.
The drawback of the approaches exploiting symmetrical structures is a higher ohmic resistance due to presence of multitude of mode-expanding layers, and higher optical loss due to expansion of the optical mode into n- and especially p-doped cladding layers.
High-power, long-cavity semiconductor lasers frequently use an asymmetric structure design, which has asymmetric optical mode distribution with more optical field in the n-side of the laser structure and less optical field in the p-side. The reason for shifting the optical mode towards the n-doped layer is that an optical loss in a laser diode mainly occurs in a p-cladding due to a much higher absorption coefficient in a p-doped material as compared to an n-doped material. For instance, in an article by B. Corbett, I. Kearney, P. Lambkin, J. Justice, U. Buckley, and K. Thomas, entitled “Engineering of InGaAsP layer structures for low divergence long wavelength lasers”, Electron. Lett., vol. 38, pp. 515-516, 2002, the authors describe incorporating two high-index InGaAsP mode-pulling layers in the active region of a laser diode, for expanding and shifting the optical field towards the n-side of the p-n junction. Further, in a U.S. Pat. No. 6,882,670 by Buda et al., a diode laser is described, in which a refractive index profile of the layer stack is made asymmetric, so as to generate an optical field distribution skewed towards the n-type layers. The asymmetry is caused by a trap layer, located in n-cladding, for attracting the optical field, and a separation layer disposed between the active layer and the trap layer, for repelling the optical field. Yet further, in a U.S. Pat. No. 6,987,788 by Kim et al., a high power semiconductor laser device is described comprising p- and n-type cladding layers having the same refractivity, and additional high refractivity layer disposed in the n-type cladding layer, for pulling the majority of the optical mode towards the n-doped cladding. All the articles and patents mentioned in this paragraph are incorporated herein by reference.
The confinement factor is a measure of the overlap of the laser gain region with the optical mode of the optical waveguide. The asymmetric design effectively reduces the internal loss and enables longer cavity length designs; however, pushing more optical mode to the n-side has some drawbacks. One is that the confinement factor can be too low, which leads to a significantly higher threshold value that could in turn result in less reliable performance. Another problem is that pushing the optical field into the n-side can create a leaky mode into the substrate which can cause significant loss and other performance degradation, such as coherent interference with abnormal oscillating spectral behavior.
An important characteristic of a semiconductor laser is its beam parameter product defined as the far-field divergence multiplied by the near-field spot radius. The prior art approaches, relying on near-field mode expansion to reduce the far-field divergence, do not substantially improve the vertical beam parameter product of the output beam. A cylindrical microlens could be used to achieve a similar result. Reducing far-field divergence without having to expand the mode size is possible and, moreover, highly desirable; however, in order to achieve that, one would have to improve the quality of the laser beam itself. Further, with either designs, symmetric or asymmetric, it is difficult to tune the divergence of the output laser beam, so as to arrive at an optimized value of said divergence, due to a multitude of layers and deposition parameters involved. A method is required to control the far-field divergence and lower it, without a considerable near-field mode expansion, so as to improve the overall laser beam quality and ease the task of coupling the beam into an optical fiber.
It is, therefore, an object of the present invention to provide a method of manufacture of a semiconductor laser device having a high output optical power, of the order of 1 W, and a very low far field divergence, of the order of 12 to 20 degrees; a method allowing one to lessen the far field divergence of output radiation of said semiconductor laser device while keeping the optical loss and associated lasing threshold increase to a minimum. Surprisingly, the method of the present invention allows one to obtain an improvement of the overall laser beam quality as a result of considerable far-field divergence reduction, with a simple adjustment of only one process parameter.